When drilling in rock or concrete, the problem of performing pilot perforations is normally solved by using a drilling tool on which there is fixed a bit that follows a guiding borehole or pilot borehole. This method, however, cannot be implemented with cohesionless soil. In fact, if the excavation requires a reiteration of ascents and descents of the tool, there is a risk that the guiding borehole may become obstructed due to partial or total collapse of the walls of the borehole, or by the fall of debris not collected by the tool. In this case, the pilot borehole becomes filled with loose material, potentially causing the tip of the tool to exit from the guiding borehole. In addition, in soft soils, the soil surrounding the guiding borehole may not effectively counter lateral forces tending to move the tool from its defined trajectory.
US 2010/0108392 A1 describes a method for the construction of large vertical boreholes and underground cut-off walls made of piles. A drilling rig with double rotary heads drives a small diameter (between 50 mm and 400 mm) drill string, and a much larger diameter drill string, which is concentric with the smaller drill string and has an annular drill bit at a lower end thereof. A steerable “mud motor” drill is provided on a lower end of the smaller drill string to make a borehole as vertical as possible. The outer drill string is advanced, enlarging the borehole and using the inner drill string as a verticality guide. This method is known for small diameter drillings. In the case of larger diameters, however, problems arise due to the great sizes and weights, which make the procedure much more difficult to implement. Such methods, therefore, require considerable modifications of commercially available machines which are commonly used for making large diameter piles.
The construction of large diameter piles typically involves the use of a bucket rigidly connected to a telescopic rod (Kelly bar) that drives and rotates the bucket. The excavation is performed by means of the reiteration of an excavation step, during which the bucket is lowered into the hole and digs by filling with the excavated soil, and a step of emptying the bucket, during which the bucket is extracted from the borehole and emptied. The two steps are repeated until the prescribed depth of the borehole is reached.
Due to the clearance between the parts of the bucket-Kelly bar system, drilling of piles typically leads to deviations from the vertical up to 2%. This limit is set in European Standard EN 1536. For those cases where the piles are meant to withstand vertical loads, this deviation does not involve particular problems. However, in case the piles are used to create a waterproof underground cut-off wall or bulkhead, or where the piles must be set side to side, this limit can create significant problems, giving rise to defects in the overall geometry of the underground wall.